Tobacco expansion processes are used to restore tobacco bulk, density, and/or volume which are lost during curing and storing tobacco leaf. In recent years, expanded tobacco has become an important component of many reduced tar cigarettes, including low tar and ultra-low tar cigarettes.
Processes in which tobacco is contacted with an expansion agent and rapidly heated to volatilize the expansion agent and expand the tobacco are described, for example in U.S. Pat. Nos. 3,524,451 to Fredrickson et al. and 3,524,452 to Moser, et al. A process employing a vapor state impregnation of tobacco followed either by heating or rapid pressure reduction for tobacco expansion is disclosed by U.S. Pat. No. 3,683,937 to Fredrickson et al.
The use of carbon dioxide as an expansion agent for expanding tobacco is disclosed in U.S. Pat. No. 4,235,250 to Utsch; U.S. Pat. No. 4,258,729 to Burde et al.; and U.S. Pat. No. 4,336,814 to Sykes et al., among others. In these and related processes, carbon dioxide, either in gas or liquid form is contacted with tobacco for impregnation and thereafter the impregnated tobacco is subjected to rapid heating conditions to volatilize the carbon dioxide and thereby expand the tobacco.
U.S. Pat. No. 4,531,529 to White and Conrad describes a process for increasing the filling capacity of tobacco, wherein the tobacco is impregnated with a low-boiling point and highly volatile expansion agent, such as a normally gaseous halocarbon or hydrocarbon at process conditions above or near the critical pressure and temperature of the expansion agent. The pressure is quickly released to the atmosphere so that the tobacco expands without the necessity of the heating step to either expand the tobacco or fix the tobacco in the expanded condition. The pressure conditions of this process range from 36 kilograms per square centimeter (512 psi) and higher with no known upper limit.
U.S. Pat. No. 4,554,932 to Conrad and White describes a fluid pressure treating apparatus, including a cylindrical shell and a reciprocating spool assembly mounted for movement between a loading position outside the shell and a treating position within the shell. Sealing members on the spool assembly are provided for engaging the shell to form a pressure chamber. Conduits are provided to introduce processing fluids into the pressure chamber. This system provided an apparatus for use in high pressure materials treatment, such as tobacco impregnation for expansion, permitting easy loading and unloading and minimizing the time associated with sealing and locking steps normally used in high pressure treatment apparatus.
U.S. Pat. No. 5,076,293 to Kramer is directed to a process and apparatus for treatment of tobacco material and other biological materials having a mechanism for forming a dynamic seal in which cooperating moving surfaces seal a treatment chamber. The dynamic seal system provided according to this patent is useful in treating tobacco at elevated temperature and pressure conditions, including conditions of supercritical temperature and pressure for processes including tobacco expansion. Both continual and batch processes are disclosed.
Tobacco expansion processes, including those described above and others, typically must be conducted in batch processes when impregnation pressures substantially above atmospheric pressure are used. To reduce cycle time for such batch processes, it is necessary to have a generally continuous supply of the impregnating agent at the appropriate high pressure and heated to an appropriate high temperature. However, some highly effective tobacco expansion processes use flammable expansion agents such as ethane or propane at high pressure and high temperature conditions.
A particularly effective process and apparatus for forming a tobacco batch is described in U.S. patent application Ser. No. 08/673,985 filed Jul. 1, 1996, by Ed Lovette and Wayne Detwiler, now U.S. Pat. No. 5,647,382, issued Jul. 15, 1997. According to that apparatus and process, tobacco is pneumatically delivered to a batch forming chamber wherein it accumulates against an abutment wall of the chamber. A volumetric sensor senses when a predetermined volume of tobacco has accumulated and the tobacco batch is delivered to a vertically oriented conduit having a separable zone. The separable zone provides a sealing conduit wherein the batch forming chamber is sealed before the tobacco is released from the separable zone and delivered to a below-positioned tobacco impregnation apparatus. This prevents any expansion agent from entering the pneumatic tobacco delivery system wherein the pneumatic system circulates gases.
In exemplary techniques, the impregnated tobacco is expanded simply by evacuating expansion agent from the pressurized impregnation chamber and thereafter exposing the impregnated tobacco to atmospheric pressure. The expanded tobacco is thereafter dried as noted above to maximize the retained filling capacity. However, prior techniques and equipment used to dry the expanded tobacco have required costly apparatus and process controls.
Further, although the majority of expansion agent can be evacuated from the impregnation chamber before exposing the tobacco to expansion conditions, some expansion agent is released from the impregnation chamber and from the tobacco during and/or after the tobacco expansion step. Accordingly, to minimize the hazards associated with flammable expansion agents, measures are normally taken to control leakage of expansion agent from impregnation chamber. In addition, because expansion agent is released from the tobacco cellular structure during expansion, measures are also normally taken to ensure removal of the released expansion agent, which may be flammable, from the expanded tobacco prior to storage of the tobacco in a closed storage area. Conventional leakage control and ventilation processes and apparatus for tobacco expansion agent, and also conventional processes and apparatus for purging and disposal of expansion agent released from the impregnated tobacco and impregnation chamber during expansion, however, have required costly apparatus and/or complex process controls.